codenothing/css-compressor — Issue #9:  breaking css url() paths with uppercase characters (@font-your-face)
Author: mikeytown2
Created: 2011-12-02 02:27:32+00:00
State: open

I think the code is in the CSSCompression_Individuals class is causing the issue.
http://drupal.org/node/1324944

url(/sites/default/files/fontyourface/fontsquirrel/Days-fontfacekit/Days-webfont.svg#DaysRegular)
Turns into
url(/sites/default/files/fontyourface/fontsquirrel/days-fontfacekit/days-webfont.svg#daysregular)